Industrial truck



Oct. 18, 1966 w. H. BRUNS 3,280,397

INDUSTRIAL TRUCK Filed March 25, 1964 4 Sheets-Sheet l CHARGER 24 D ANDSPEED AND PUMP 4| REVERSING 1 REVERSI MOTOR SWiTCH g; SWITCH e l l l 2542 k ACCESSORY 52 i l 57 32 @lZ'----- GP' 43 58 i l 26 e5 LIQUID 59 W/LLL41? HEMQYEEUMS l N V E N TO R BY Vim ATTORNEY Oct. 18, 1966 w. H.BRUNS INDUSTRIAL TRUCK 4 Sheets-Sheet 5 Filed March 25, 1964 W/LLlflN/JENKYBAUNS IN VE NTOR BY mi M ATTORNEY Oct. 18,1966

Filed March 25, 1964 W. H. BRUNS INDUSTRIAL TRUCK 4 Sheets-Sheet 4VOLTAG E VVVV l l w W GASOLINE ENGINE mum ymeyszu/vs INVE NTOR BYWZWATTORNEY United States Patent 3,280,397 INDUSTRIAL TRUCK William HenryBruns, Lincolndale, N.Y., assignor to Otis Elevator Company, New York,N.Y., a corporation of New Jersey Filed Mar. 25, 1964, Ser. No. 354,64712 Claims. (Cl. 318 -139) This invention relates generally to fork lifttrucks and particularly to a novel system for controlling the speed ofboth the truck itself and the hoist associated therewith.

Small self-propelled industrial trucks are widely used for handlingmaterial in warehouses. The usual truck includes a motor for propellingthe truck and a power operated hoist, often with a pair of forks as theplatform, for raising and carrying the load. Power plants of variouskinds, such as gasoline, electric, and combinations thereof have beenused. One of the most common arrangements comprises a pair of seriesmotors, one for propelling the truck and one for operating the hoist,both supplied with electricity from a battery mounted on the truck. Inthis arrangement, the propulsion motor is geared to the Wheels and therunning speed is controlled by varying the amount of resistanceconnected in series with the motor. A friction brake is used todecelerate the truck for stopping and to control the speed on downgrades. Typically the hoisting motor drives a pump which forces fluidinto a cylinder to raise the load. The load is lowered by a valve whichcontrols the rate at which fluid in the cylinder is released to areservoir.

The above and other similar arrangements have certain disadvantages. Forexample, with any particular setting of the speed controller, theresulting speed varies widely with different torque requirements due tothe inherent operating characteristics of a series motor. For thepropulsion motor, torque requirements depend upon the weight of the loadand the grade encountered. For the hoisting motor, torque requirementsdepend upon the load on the forks. In order to obtain any given speedthe operator must adjust the controller in accordance with differentload conditions.

Another disadvantage is that the above-described arrangement wastespower. Whenever either motor is running at less than full speed, poweris dissipated in the series resistors. Whenever there is a negativeload, as when the truck is decelerating or going down hill or when theload on the fork is being lowered, the kinetic and/or potential energyis dissipated either in the friction brake or in the hydraulic loweringvalve.

A fork lift truck in normal operation is starting or stopping, andraising or lowering a load practically continuously. A large proportionof the energy drawn from the battery does no useful work but is wasted.As a result, the battery must either have a very large capacity or berecharged frequently. Truck mounted battery chargers have beenunattractive because of the size and cost of a charger of sufficientcapacity to replenish the large amount of power withdrawn.

It is a general object of the present invention to provide an improvedindustrial truck.

Another object is to improve the running speed control of an industrialtruck.

A more specific object is to provide an industrial truck with a runningspeed control system by which an operator can select a desired speed,which speed is substantially independent of either the grade, whether upor down, or the load being carried.

Another object is to provide a fork lift truck with improved control ofthe hoisting and lowering speed.

Anotherobject is to provide a system for controlling the hoist of a lifttruck by which an operator can select a desired hoisting or loweringspeed which is substan- I 3,280,397 Patented Oct. 18, 1966 tiallyindependent of the weight of the load on the hoist.

Another object is to provide a battery powered electric truck in whichthe power drawn from the battery is used efliciently.

Another object is to provide a battery powered truck which requires buta small battery.

Another object is to provide an improved charging system for the batteryof an electric truck.

Briefly stated, the invention comprises two fixed excited electricmotors, one for propelling the truck and the other for operating thehoist, both supplied with power from a multi-celled battery mounted onthe truck. The battery is tapped to make available several differentvoltages. The running speed of each motor is determined by its ownswitch controller which selects the desired portion of the total batteryvoltage and applies it to the armature. Each position of the controllercorresponds to a preselected motor speed which, because of the inherentoperating characteristics of a motor with fixed field excitation, variesvery little with the torque requirements. When the load tends to driveone of the motors, as when going down grade, decelerating, or loweringthe load on the hoist, the motor acts as a generator and charges thebattery.

The invention also contemplates a charging system which may operateeither continuously or intermittently. In either case, each group ofcells between taps is charged from a source of alternating current thepeak value of which is regulated to be substantially equal to the fullycharged potential of the cells and which is applied to the cells througha rectifier. Thus, although the cells may discharge at different rates,the charging rate is automatically accommodated to the condition of thecells and overcharging is prevented.

For a clearer understanding of the invention reference may be made tothe following detailed description and the accompanying drawing, inwhich:

FIGURE 1 is a simplified pictorial view of a lift truck;

FIGURE 2 is a schematic diagram illustrating the principles of theinvention;

FIGURE 3 is a schematic diagram illustrating the principles of the speedcontrol switch;

FIGURES 4 to 7, inclusive are fragmentary diagrams similar to FIGURE 3showing the switch in various operating positions;

FIGURE 8 is a schematic diagram of the electrical circuits associatedwith the propulsion machine;

FIGURE 9 is a schematic diagram of one of the relays;

FIGURE 10 is a fragmentary schematic digaram of the electrical circuitsassociated with the hoisting machine;

FIGURE ll is a schematic diagram of a preferred form of a batterycharger;

FIGURES 12 and 13 are schematic diagrams of a modification of thebattery charging circuit; and

FIGURE 14 is a schematic diagram of another modification of the batterycharging circuit.

Overall description Referring first to FIGURE 1, there is shown a lifttruck 15 incorporating the invention. The truck 15 includes rearsteerable wheels 16, front drive wheels 17, uprights 18, and a platformsuch as the forks 19 on which a load may be placed for lifting ortransporting. The elements comprising the invention, although notvisible in FIG. 1, are all mounted on the truck 15 and will be describedin connected with the remaining figures of the drawing.

Referring now to FIGURE 2 there is shown a simplified schematic diagramof the principal elements of the invention all of which may be mountedon the lift truck 15. There is shown a battery 21 for supplying thenecessary power which comprises a number of cells connected in seriesand having taps connected at various intermediate points to provide asource of variable voltage. A dynamo electric machine 22 includes anarmature 23 and a field winding 24 which is separately, or fixed,excited by being connected to an intermediate tap of the battery 21. Themachine 22 is mechanically connected through a gear box 25 to amechanical differential 26 which, in turn, is connected to the drivewheels 17. A mechanical brake is shown schematically as comprising abrake drum 31 fastened to the shaft between the gear box 25 and thedifferential 26 and a brake shoe 32 applied by a foot pedal 33. A switch34 actuated by the pedal 33 is closed when the brake is released and isopen when the brake is applied. The machine 22 operates as a lIIlOtOI todrive the wheels 17 unless the load overhauls the machine in which caseit acts as a generator.

A speed and reversing switch 35, to be more fully described, enables anoperator to select various voltages from the taps of the battery 21 tobe applied to the armature 23 to control the speed of the truck. Thespeed versus torque characteristics of a D.C. machine with fixedexcitation has a small slope which means that the speed is dependentalmost entirely on the voltage applied to the armature and depends verylittle on the torque requirements. In comparison with a series motor, afixed field motor may be regarded as operating at a substantiallyconstant speed for any given armature voltage, regardless of the torquerequirements, whether positive or negative.

Hoisting is controlled by a similar dynamo electric machine 37comprising an armature 38 and a field winding 39 the latter also beingconnected to an appropriate intermediate tap of the battery 21. Themachine 37 is mechanically connected to a positive displacement pump 41,more properly described as a pump-motor or as a pump-motor devicebecause it is capable of being driven in either direction by the machine37 to act as a pump and is also capable of acting as a motor to drivethe machine 37 in either direction. One side of the device 41 ishydraulically connected through a valve 42 to an accumulator 43 whichmay, for example, be a closed vessel with an air-liquid interface. Theother side of the device 41 is hydraulically connected to a cylinder 44which operates a piston rod 45. The piston rod 45 is mechanicallyconnected to a sprocket 46 over which passes a chain 37 one end of whichis fixed and the other end of which is connected to the platform 19 onwhich a load 49 may be placed in order to be raised or lowered. A speedand reversing switch 51 interconnects the armature 48 and the varioustaps of the battery 21 to control the speed. This switch may be similarto the switch 35 except that it also includes an auxiliary contact whichis open when the switch is in the Zero speed position but which isclosed at all other times. This switch, when closed, energizes asolenoid 52 which opens the valve 42. When the solenoid is deenergizedthe valve is spring-biased to its closed position. It is to beunderstood that the electrical, mechanical and hydraulic connectionsshown in FIGURE 2 are schematic only and are simplified versions ofthose which would actually be used on a commercial truck.

A battery charging system 55 is connected to each tap of the battery 21so that each group of cells may be charged separately while non areovercharged. The charger preferably includes a truck mounted gasolineengine and generator and may also include provision for plugging into anelectrical outlet. A suitable charger will be more fully describedsubsequently.

The truck is moved by releasing the brake and actuating the switch 35 toconnect the lowest voltage tap of the battery 21 to the armature 23. Themachine 22 operates as a motor and quickly accelerates the truck to thespeed corresponding to this tap. To go faster, a higher voltage tap isselected and the truck quickly accelerates to the new speed. If thetruck encounters an up grade, the speed tends to fall slightly, reducingthe counter electromotive force of the machine 22 thus increasing thearmature current and thereby preventing any substantial decrease inspeed. If the truck encounters a down grade, the speed tends toincrease, whereupon the machine 22 acts as a generator, providingregenerative braking and recharging of the battery. To reduce speed, theoperator selects a lower voltage tap. When the battery voltage is firstreduced, the counter electromotive force is much larger than the batteryvoltage and a large charging current is supplied to the battery. Thus,regenerative braking quickly decelerates the truck to the speedcorresponding to the newly selected tap. It is apparent that eachvoltage tap selected corresponds to a predetermined speed which speedremains substantially constant regardless of whether the truck is goingup grade or down grade. The truck may be stopped by actuating the switch35 to the zero voltage tap which substantially short circuits thearmature 23 quickly halting the truck. The truck may be held stationaryby depressing the brake pedal 33.

The hoist is operated by actuating the switch 51 to the first hoistingspeed position. The valve 42 is opened, the machine 37 is energized andthe pump-motor device 41 is operated. If a very heavy load is beinglifted, the machine 37 acts as a motor, the device 41 operates as a pumpand pumps liquid from the accumulator 43, which now constitutes a sourceof fluid, to the cylinder 44. The connecting rod 45 raises the sprocket46, the platform 19 and the load 49. To stop the hoist, the switch 51 isreturned to the zero speed position at which the armature 38 issubstantially short circuited and the valve 42 is closed. The load maybe lowered by actuating the switch 51 to, for example, its firstlowering speed position. The valve 42 again opens. If the load is veryheavy it drives the rod 45 downward, fluid drives the device 41 as amotor, and the machine 37 is driven as a generator thereby charging thebattery and providing regenerative braking. At the same time fluid ispumped into the accumulator 43, thereby further compressing the trappedair and storing energy.

If a lighter load is to be raised the switch 51 may again be actuated toits first hoisting speed position. The pressure in the accumulator 43may be sufficient to drive the device 41 as a motor thereby raising theload 49, driving the machine 37 as a generator and charging the battery.In this case energy previously stored in the accumulator is expendedboth to raise the load and to charge the battery. To lower a light loadthe switch 51 is actuated to one of its lowering speed positions. Themachine 37 acts as a motor driving the device 41 as a pump to transferfluid from the cylinder 44 to the accumulator 43. In this case energy istransferred from the battery to the accumulater.

It is apparent that during a number of hoisting and lowering cycles,energy is automatically transferred among the battery, the accumulatorand the load. No energy is uselessly dissipated in resistors, valves ormechanical brakes, or wasted by allowing fluid to return to a reservoir.The net energy expended is only that required to do useful work and tosupply the inherent losses in the system.

In addition to the propulsion and hoisting systems, lift trucks areoften provided with a number of power operated auxiliary devices such asdevices to shift the position of the forks and devices to clamp a load.Such devices are herein termed accessories. One accessory is shownschematically in FIGURE 2 by the rectangle 56, mechanically connected toa piston rod 57 actuated by a hydraulic cylinder 58. The cylinder 58 isconnected through a valve 59 to a hydraulic line 61 which, in turn, isconnected to the accumulator 43. The valve 59 may be a conventionalthree way valve. In its first position all ports are closed. In itssecond position liquid from the line 61 is admitted to the cylinder 58.In its third position liquid is allowed to escape from the cylinder 58through the line 62 to a sump or reservoir 63.

The accumulator 43 is maintained charged and the reservoir 63 preventedfrom overflowing by an auxiliary pumping system. A pump 64 driven by anelectric motor 65 draws liquid from the reservoir 63 and pumps itthrough a check valve 66 to the line 61 and thence to the accumulator43. A float 67 is mechanically connected to close a switch 68 when theliquid in the reservoir 63 reaches a predetermined level. Closure of theswitch 68 completes a circuit from a source of power such as a battery69 to the motor 65 which drives the pump 64 thereby reducing the liquidlevel in the reservoir 63 and charging the accumulator 43.

By the above arrangement there is always enough energy stored in theaccumulator to operate the various accessories without energizing thehoisting motor 37. The energy consumption of the accessories is smalland it has been deemed unnecessary to attempt to recover any of theenergy expended in their operation.

Speed changing switch The batteries ordinarily used in industrial trucksconsist of many cells connected in series and accordingly there isavailable a wide range of voltages. It would appear at first glance thatit would be possible to eliminate the starting resistors simply byapplying various voltages to the armature by means of taps connected tovarious numbers of cells of the battery. Upon analysis, however, it isfound that this approach is not as simple as it would at first appear.The difliculty is two fold. First, it is necessary, to avoid shortcircuiting any cells of the battery during tap changing because theresulting currents would be so large as to damage the battery and theassociated electrical circuits. Therefore, a simple make before breakswitch cannot be used. Second, the armature constitutes a largeinductance which may be carrying a substantial current. If this circuitwere broken, the high voltage induced by the sudden change in currentcould cause severe damage to the equipment. Additionally, if thearmature circuit were broken, the motor would in effect be disconnectedfrom the truck and speed control would be lost. If the truck were goingup hill, the speed would fall suddenly while if the truck were goingdown hill, the speed would rise suddenly. Therefore, a simple breakbefore make switch cannot be used. According-1y, it is an object of theinvention to provide a switch arrangement for applying the variousvoltages of a multicelled battery to a DC. motor without either shortcircuiting any battery cells or opening the armature circuit duringvoltage changes.

Referring now to FIGURE 3, there is shown the multicelled battery 21with opposite ends connected to terminals 71 and 72. A plurality of taps73-79 inclusive are provided, the tap 73 being connected to the terminal71, the tap 79 being connected to the terminal 72 and the remaining tapsconnected to various intermediate points on the battery. The differencein potential between adjacent taps may be equal or unequal dependingupon the requirements of the particular application. The terminal 71 isconnected to a common or ground conductor 81.

The electric motor 22 is shown mechanically connected to a load 83 suchas the driving wheels of an industrial truck. The field winding 24 hasone terminal connected to the common conductor 81 and the other terminalconnected to one of the battery taps such as the tap '75.

A principal contact 84 makes sliding contact with the taps 7379 and isconnected to one terminal of the armature 23 the other terminal of whichis connected to the common conductor 81. Two auxiliary contacts 85 and86 are positioned on either side of the principal contact 84 and aremechanically connected thereto as indicated schematically by the dashedline 87. All three contacts are movable as a unit, up or down as viewedin FIGURE 3, as indicated by the arrows 88 and 89. One of the auxiliarycontacts leads the position of the contact 84 while the other lags. Asthe contacts are moved upward, as viewed in FIGURE 3, the contact 86leads and the contact S5 lags. As the contacts are moved downward, thecontact leads while the contact 86 lags.

The contact 86 is connected to the cathode of a diode 91 the anode ofwhich is connected to the contact 84. The contact 85 is connected to theanode of a diode 92 the cathode of which is connected to the contact 84.Each of the diodes 91 and 92 may be any suitable kind of unidirectionalconducting device, or rectifier, such as an electron tube, or agermanium or silicon rectifier, or other solid state device. The termdiode is intended to comprehend any such device.

The relative widths and spacings of the taps 73-79 and the contacts84-86 are selected so that as the contacts are moved (1) each contactdisengages a tap before engaging the next succeeding tap, and (2) twoadjacent contacts can engage the same tap simultaneously. A simplearrangement is illustrated wherein the spacing between taps is equal tothe width of each tap, each contact is approximately one-half the widthof a tap and the spacing between the center lines of adjacent contactsis approximately equal to the width of a tap. Obviously there are otherarrangements of taps and contacts which can be used to accomplish thesame result.

FIGURE 3 illustrates the position of the parts when the motor is runningat an intermediate speed. Current flows to the armature 23 through thetap 76 and the contact 84. The contacts 85 and 86 do not engage any tapat this time. If it is desired to increase the speed the contacts aremoved'upward. The first change in connections is illustrated in FIGURE 4wherein contacts 84 and 85 are both engaging the tap 76 and contact 86is engaging the tap 77. Most of the armature current flows through thecontact 84 although a small amount may flow through the contact 85 andthe diode 92. Although the contact 86 is engaging the tap 77, the diode91 prevents current from flowing from the tap 77 to the tap 76. FIG- URE5 illustrates the next change in connections. The contact 84 is not inengagement with any tap but armature current flows from the tap 76through the contact 85 and the diode 92. As before, the diode 91prevents current from flowing from the tap 77 to the tap 76. FIGURE 6illustrates the next change in connections. The contact 85 engages thetap 76 while contacts 84 and 86 bot-h are engaging the tap 77. Armaturecurrent now flows from the tap 77 through the contact 84. The diode 92now prevents the flow of current from the tap 77 to the tap 76. FIGURE 7illustrates the connections when the next speed has finally beenselected. The contacts 85 and 86 are disengaged while the contact 84engages the tap 77. The motor is now operating at a higher speed. It isto 'be noted that throughout the tap changing operation the armaturecircuit was never broken and adjacent taps were never short circuited.

When it is desired to decelerate the motor, the sequence of events isreversed. Assume first that the machine 22 is acting as a motor, as itwould if the truck were going up hill, and that the switch is in theposition shown in FIGURE 7. The counter electromotive force of the motoris less than the voltage of the tap 77 by an amount equal to the IR dropin the armature and is greater than the voltage of the tap 76. Thecurrent that flows from the tap 77 through the contact 84 to the motoris the amount required to keep the truck running at a constant speed.When the switch is moved to the position shown by FIG- URE 6, there isno change. Current continues to flow from the tap 77 through the contact84 to the motor. But when the position of FIGURE 5 is reached,conditions change. Current cannot flow from the tap 77 to the motorbecause of the diode 91. The counter electromotive force is not largeenough to cause a current to fl ow through the diode 91 to the tap 77.The voltage of the tap 76 is less than the counter electromotive forceso that current does not immediately flow from left to right through thecontact 85 and the diode 92 and current cannot flow from right to leftthrough the diode 92. Current is thus reduced. This change in currentcauses a voltage to be induced in the armature due to its inductancewhich voltage is in a direction tending to perpetuate the flow ofcurrent. This induced voltage is in opposition to the counterelectrornotive force and is added to the battery voltage. Since the sumof the voltage drops in the closed circuit from the tap 76, through thediode 92, the motor armature, and the battery must be equal to zero, andsince the counter electrornotive force remains momentarily at its formervalue which is the voltage of tap 77 less the IR drop in the armature,the induced volt-age rises only until it is equal to the difference inpotential between the taps 77 and 7 6, whereupon current flows from thetap 76 through the contact 85 and the diode 92 to the motor. In otherwords, the induced voltage can never exceed the voltage between adjacenttaps, and a smooth transition occurs. When the switch reaches theposition shown in FIGURE 4, current can flow between the tap 76 and thecontact 84 in either direction, depending upon the magnitude of thecounter electromotive force which depends upon the speed of the machine22.

Assume now that the switch is in the position shown in FIGURE 7 and thatthe machine 22 is acting as a generator as it would, for example, if thetruck were going down grade. Current then flows through the contact 84and the tap 77 to the battery. As the switch is moved to the position ofFIGURE 6, current flows as before. In the position of FIGURE 5, currentflows through the diode 91 and the contact 86 to the tap 77. In theposition of FIGURE 4, current flows through the contact 84 and the tap76 to the battery.

It is thus apparent that the system above described enables the armatureof a dynamo electric machine to be connected successively to varioustaps of a multi-celled battery. No battery cells are ever shortcircuited. Large induced voltages and destructive arcing are eliminatedbecause the armature circuit is-never opened.

Propulsion motor circuit Referring now to FIGURE 8, there are shownfurther details of the electrical connections of the battery 21, thedynamo electric machine 22, and the speed control and reversing switch35. The latter comprises a speed control switch shown in the upper partof the figure and designated generally by the reference character 101,and a reversing switch shown in the lower left and designated generallyby the reference character 102. Both are shown schematically 'as drumcontroller switches. The switch 101 comprises a plurality of contactfingers 103-112, inclusive and a plurality of contact segments 115-119,inclusive. The segments 115, 116 and 117 correspond to the segments 84,85 and 86, respectively of FIGURES 3-7, and each comprises a pluralityof segments electrically connected as shown. The segment 115 isconnected directly to the segment 118 while the segments 116 and 117 areconnected to the segment 118 through the diodes 92 and 91, respectively.The reversing switch 102 comprises a plurality of contact fingers suchas the finger 121 and a plurality of contact segments 122-128,inclusive.

Also shown in FIGURE 8 are the operating windings F1 and F2 of tworelays, connected in series with the armature 23, and provided withcontacts F1-1 and F2-1, respectively. A diode 131 is connected acrossthe field winding 24 to provide a discharge path when the energizingcircuit is broken. A relay F3 is provided with two operating windingsF3A and F3B and with two contacts F3-1 and F3-2. The relay F3 preferablycomprises two separate solenoids, as shown schematically in FIGURE 9,mechanically connected together and arranged so that either winding whenenergized will operate the relay and the contacts F3-1 and F3-2.

The remainder of the circuit of FIGURE 8 can best be described byconsidering the operation. To move the truck forward, the reversingswitch 102 is moved to its FWD position. The contact segment 122partially completes a circuit for energizing the field Winding 24.

This circuit may be traced from that tap of the battery 21 which isconnected to the contact finger 105, through a conductor 132, theanode-cathode circuit of a diode 133, the segment 122 and the brakeswitch 34. When the latter switch is closed, the circuit is completedthrough the field winding 24 to the ground conductor 81 and then to thebattery terminal 71. At the same time, the armature 23 is substantiallyshort circuited, the circuit including the low resistance windings F1,F2 and F313. The circuit may be traced from the upper armature brushthrough the windings F2 and F1, the segment 123, a conductor 134, thefinger 110, the segment 118, the segment 115, the contact finger 103,the terminal 71, the ground conductor 81, the winding F3B, and thesegment 124 to the lower brush of the armature 23. Next, the switch 101is moved to its first position. The segment 119 completes a circuit fromthat tap of the battery which is connected to the finger 105, throughthe fingers 111 and 112, a conductor 135, the normally closed contactsF3-2, the winding F3A, and the ground conductor 81 to the batteryterminal 71. The contacts F3-1 close, bridging the switch 34 and thecontact fingers associated with the segment 122 so that the fieldwinding 24 remains energized even if one of these circuits is opened. Assoon as the relay F3 operates, the normally closed contact F3-2 opens,inserting a resistor 136 in series with the winding F3A (to reduce thecurrent and consequent heating because less current is required to holdthe relay than to operate it).

The sequence of engagement of the segments 115, 116 and 117 during thetransition from the zero position to the first speed position isidentical to that previously described for contacts 84, and 86 inconnection with FIGURES 3-7, inclusive and need not be repeated. Whenthe first speed position is reached, the armature is energized by acircuit which can be traced from the first tap of the battery 21, thecontact finger 104, segment 115, segment 118, contact finger 110,conductor 134, segment 123, windings F1 and F2, the armature 23, segment124, winding F3A and the conductor 81 to the battery terminal 71. Thetruck moves forward. Higher speeds are obtained by moving the switch 101to higher speed positions which increase the voltage applied to thearmature 23.

The curve of torque as a function of armature current for a DC. motorwith constant field excitation is approximately linear for aconsiderable range of armature currents. But as the armature current isincreased further and further, the curve tends to flatten out. In thisregion a very large increase in armature current is required in order toobtain a small increase in torque. The excitation of the field winding24 from the contact finger is selected so that for normal loads,operation occurs in the approximately linear portion of the curve.However, to obtain the high torque required for unusually heavy loads,it is preferred to increase the field excitation. Such an increase hasthe effect of increasing the slope of the curve and extending theapproximately linear portion to higher torques.

An increase in field excitation is obtained by means of the previouslymentioned relays F1 and F2. If a heavier than normal load isencountered, the speed tends to decrease, thereby increasing thearmature current. At a predetermined current, the relay F1 operates,closing its contacts F1-1 thereby increasing the available torque byincreasing the field excitation. A circuit may now be traced from thatbattery tap connected to finger 106, a conductor 137, the anode-cathodecircuit of a diode 138, and the contacts F1-1 to the field winding 24.The diode 133 prevents short circuiting of adjacent battery taps. If thetorque now available is not sufiicient, the armature current increasesfurther until a second predetermined value is reached at which time therelay F2 operates closing its contacts F2-1 thereby further increasingthe field excitation through a conductor 139 to that tap connected tothe finger 107. The diode 138 now also acts, along with the diode 133,to prevent short circuiting of adjacent cells. As the load decreases,the current through Windings F2 and F1 decreases sufficiently to releasethe contact F2-1 and then the contact F1-1, restoring normal fieldexcitation.

An increase in field excitation is, of course, accompanied by a decreasein speed. It is preferred, under unusually heavy load conditions, tosacrifice substantially constant speed for each setting of the speedcontroller in the interest of limiting the armature current toreasonable values and minimizing the drain on the battery. In the hightorque region of operation the total power drawn from the battery forboth the armature and field is substantially less when the fieldexcitation is increased as above described.

To decrease the speed of the truck, the switch 101 is actuated to alower speed position. The counter electromotive force then exceeds theapplied voltage and the machine 22 acts as a generator thereby chargingthe battery and quickly decelerating the truck. To stop the truck, theswitch 101 is actuated to its zero speed position. The contact segment119 disengages the contact finger 111 thereby deenergizing the relaywinding F3A. However, the contact F3-1 does not open because the windingP38 is in the armature circuit, as previously pointed out, and theregenerative braking current therethrough holds the contact F3-1 closed,thereby maintaining the field excitation. The truck is stopped quicklyand may be held by applying the mechanical brake.

Hoisting motor circuit The electrical circuit for the hoisting machine37 may be identical to that of the propulsion machine 22 except for thecircuit controlling the valve 42 which requires additional contacts onthe speed control switch. Referring to FIGURE 10, there is shown afragmentary view of a speed control switch, denoted generally by thereference character 151. The parts which are identical to those ofFIGURE 8 are denoted by primed reference characters corresponding tolike unprimed reference characters in FIGURE 8. There are shown inFIGURE the contact finger 112' connected to the conductor 135 and afragment of the contact segment 119'. Also shown are two additionalcontact fingers 152 and 153 which cooperate with a contact segment 154.The finger 152 is connected to the conductor 132 (which, it will berecalled, is connected to an intermediate tap of the battery). Thefinger 153 is connected by a conductor 155 to one terminal of thesolenoid 52, the other terminal of which is connected to the common orground conductor 81. When the switch 151 is in the off position, thesolenoid is not energized and the valve 42 is held in its closedposition by an internal spring (not shown). When the switch 151 isactuated to any position other than the off position, the contactsegment 154 engages contact finger 152 thereby energizing the solenoid52 and opening the valve 42.

The battery charger Referring now to FIGURE 11, there is shownschematically a preferred form of the battery charger 55. A single phasealternating current generator or alternator 161 has its output connectedthrough a double pole double throw switch 162 to the primary winding 163of a transformer 164. The transformer 164 has secondary windings equalin number to the number of sections of the battery 21. In one embodimentthe battery has six sections, 21a to 21], inclusive and the transformer164 has six secondary windings 165 to 170, inclusive. The winding 165has one end connected to the anode of a diode 173 the cathode of whichis connected to the terminal 72. The other end of the winding 165 isconnected to the junction of battery sections 21a and 21b. The winding166 is similarly connected, one end being connected to the anode of adiode 174 the cathode of which 10 is connected to the junction of thebattery sections 21a and 21b while the other end is connected to thejunction of the battery sections 21b and 210. The windings 167, 168, 169and 170 are similarly connected to battery sections 21c, 21d, 21c and 21through the diodes 175, 176, 177 and 178, respectively. The diodes arepoled, that is, the cathodes and anodes are connected, so as to preventthe flow of current from any battery section through the correspondingwinding while allowing current to flow from the winding to thecorresponding section in such a direction as to charge the battery. Forexample, with the battery polarity as shown in FIGURE 11, the terminal72 is positive and is connected to the cathode of the diode 173 whilethe anode is connected to the winding 165.

The turns ratios of the transformer 164 are selected in conjunction withthe voltage of the alternator 161 to make the peak voltage (ascontrasted with the R.M.S. voltage) of each winding equal to theterminal voltage of the corresponding battery section when fully chargedplus the voltage drop across the diode. For example, if the batterysection 21a has a terminal voltage when fully charged of 12.0 volts andthe diode 173 has a drop of 0.5 volt, the number of turns of the winding165 would be selected to make the peak voltage 12.5 volts. If all thebattery sections have the same number of cells, the windings 165 178 maybe identical. If the various sections have different numbers of cellsand, therefore, different terminal voltages, the number of turns of thevarious windings are selected accordingly. The various secondarywindings constitute, in effect, separate sources of alternating current.

When a storage battery is fully charged, it has a definite terminalvoltage. As power is withdrawn, the battery dis charges and its terminalvoltage falls. With the present arrangement the charging current isgreatest when the battery section is most fully discharged and decreasesas the battery becomes charged, falling to substantially zero when thebattery becomes fully charged. Although the various sections may bedischarged at different rates, the charging current is automaticallyadjusted in accordance with the condition of each section and no sectionis ever overcharged.

The alternator 161 is driven by a truck mounted engine 181 such as aninternal combustion engine or a gasoline turbine. The output voltage ofthe alternator 161 depends upon its speed and to keep the voltageconstant, the speed of the engine 181 is controlled by a suitablecircuit such as the circuit 182. The engine 181 is provided with athrottle, shown as comprising a shaft 183 to which an arm 184 isfastened. A tension spring 185 is connected between the arm 184 and theframe of the engine 181 to bias the throttle towards its open position.

The circuit 182 includes a voltage divider comprising a diode 186, aresistor 187, a zener diode 188 and a resistor 189 serially connected inthat order across the output conductors 191 and 192 of the alternator161. A transformer 193 has a primary winding 194 connected across theresistor 189 and a secondary winding 195 connected to a servo amplifier196. The latter controls the energization of a torque motor 197 which ismechanically connected, as shown schematically by the dashed line 198,to the throttle shaft 183 and tends to close the throttle in oppositionto the spring 185.

The zener diode 188 is selected to have a breakdown voltagesubstantially equal to the desired output voltage of the alternator 161.If the voltage is less than the desired voltage, no current flowsthrough the voltage divider and no input signal is applied to theamplifier 196. The servo amplifier 196 is designed to apply a smallcurrent to the torque motor 197 in the absence of an input signal butthis current is not sufiicient to overcome the urging of the spring 185and accordingly the throttle is opened, increasing the speed of theengine 181 and increasing the output voltage. When the out-put voltagerises to the desired value, the zener diode 188 breaks down and acurrent flows through the diode 186, the resistor 187, the zener diode188 and the resistor 189. This current is a pulsating direct current,due to the rectifying action of the diode 186. The alternating componentof the voltage drop across the resistor 189 is coupled by thetransformer 193 to the servo amplifier 196 which increases the currentapplied to the motor 197 thereby moving the throttle toward its closedposition, decreasing the speed of the engine 181 and decreasing theoutput voltage. The servo amplifier 196 has a very high gain and isprovided with the usual anti-hunt circuits so that the system is soonstabilized with the speed of the engine 181 and the output voltage ofthe alternator 161 substantially constant.

It would be possible to dispense with the transformer 164 by providingthe alternator 161 with a suitable number of separate windings. A threephase or six phase alternator could be used or the various windingscould have any other phase relationship since phase is not important inthe present invention. If such an alternator were used, the variouswindings would be connected in place of the secondary windings 165-170.However, at present it is preferred to use the system illustrated inorder to provide a convenient arrangement for charging the battery fromthe commercial power lines. The system is designed so that the outputvoltage of the alternator 161 is substantially equal to the linevoltage. To charge the battery from the line, the switch 162 is moved tothe upper position, as viewed in FIGURE 11, thereby disconnecting theprimary winding from the alternator 161 and connecting it to theconductors 201 and 202. These conductors are connected to the output ofan AC. voltage regulator 203, the input of which may be connected by theconductors 204 and 205 and the plug 206 to any convenient source. Thecharging operation is the same as previously described except that theengine 181 and the alternator 161 are not used. If the voltageregulation of the source is good enough, the voltage regulator 203 maybe omitted.

FIGURES 12 and 13 illustrate a modification of the charger previouslydescribed wherein only one-half the number of secondary windings arerequired. FIGURE 12 shows a transformer 164' having a primary winding163' which may be connected as previously described in connection withFIGURE 11. The transformer 164 is provided with a secondary winding 221having terminals 222 and 223, another secondary winding 224 havingterminals 225 and 226, and a third secondary winding 227 havingterminals 228 and 229. As shown in FIGURE 13, the terminal 222 isconnected to the junction of the battery section-s 21a and 21b. Theterminal 223 is connected to the anode of a diode 231 the cathode ofwhich is connected to the terminal 72. The terminal 223 is alsoconnected to the cathode of a diode 232 the anode of which is connectedto the junction of the battery sections 21b and 210. The secondarywinding 224 is similarly connected to diodes 233 and 234 and to thebattery sections 210 and 21d, as shown. Likewise the winding 227 isconnected to the diodes 235 and 236 and to the battery sections 21s and21 During the half cycle when the terminal 223 is positive, the batterysection 2112 is charged through the diode 231. During the next halfcycle, the terminal 222 is positive and the battery section 21b ischarged through the diode 232. Similarly the battery sections 21c and21d are charged during alternate half cycles by the current flowingthrough the winding 224 and likewise the battery sections 21c and 21 arecharged during alternate half cycles by the current flowing in thewinding 227. By this arrangement, six battery sections may be chargedusing but three secondary windings, provided that the pairs of sections,such as the sections 21a and 21b, charged by the same winding have thesame voltage.

FIGURE 14 shows a further modification. The winding 221 is provided withan intermediate tap 241 which is connected to the anode of the diode231. The cathode of the diode 234 is connected to a tap 242 on theWinding 224 instead of to the terminal 226. The winding 227 is connectedas before. By this arrangement the winding 221 can charge the sections21a and 21b when the section 21b has a higher voltage (more cells) thanthe section 21a. Similarly, the winding 224 charges the sections 210 and21d when the section 21c has a higher voltage than the section 21d.

The use of a truck mounted charger as above described makes it possibleto use the truck in remote areas where there are no power lines fromwhich to charge the battery. Unlike a straight gasoline truck, the truckof the present invention can be operated for limited periods in areaswhere exhaust fumes cannot be tolerated, simply by turning otf theengine. The previously described regenerative braking. feature reducesthe net drain on the battery so that a small engine is sufiicient forcharging. Additionally, the battery may be charged from the housecurrent whenever such .a source is available.

Summary It is apparent that the present invention provides improvedrunning speed control of both the propulsion and hoisting functions.Indeed, running speed is selected, rather than controlled, because eachcontroller position corresponds to a definite speed which issubstantially constant regardless of the load imposed. Such speedselection is made possible by the use of fixed excited motors, insteadof series motors, and by selecting the armature voltage from the widerange available from a multi-celled battery. At the same time, power isconserved by the automatic regenerative braking and battery chargingfeature. This in turn reduces the size of the battery required. Sinceless net power is consumed per hour of operation, it is feasible to usea small, truck mounted, gasoline driven battery charger. Equal chargingof all cells is obtained by the use of multiple sources of alternatingcurrent and rectifiers, with the peak voltage controlled to besubstantially equal to the voltage of the cells when fully charged.

It is to be understood that the description has omitted manyconventional details in order to avoid obscurring the invention. Forexample, it may be desirable to restrict the operation of the speedcontrol switch by dash pots or other time delay devices. Similarly,compensating motor windings have not been illustrated although their useis desirable, as is the use of conventional protective devices such ascircuit breakers.

It is also to be noted that the invention contemplates a substantiallyconstant field flux and that such a flux may be obtained with apermanent magnet field structure.

Although a preferred embodiment has been described in considerabledetail for illustrative purposes, many modifications will occur to thoseskilled in the art. It is therefore desired that the protection affordedby Letters Patent be limited only by the true scope of the appendedclaims.

What is claimed is:

1. An industrial truck, comprising,

drive wheels,

a hoisting mechanism,

a storage battery comprising a plurality of cells connected in series,

a first direct current dynamo electric machine having an armatureoperating in a substantially constant magnetic field and operativelyconnected to said wheels,

first switch means for selectively connecting various numbers of saidcells to said armature of said first machine,

said first switch means including a first principal movable contact andfirst and second auxiliary contacts leading and lagging, respectively,the position of said first principal contact and each connected througha diode to said first principal contact,

a second direct current dynamo electric machine having an armatureoperating in a substantially constant magnetic field and operativelyconnected to said hoisting mechanism, and 4 second switch means forselectively connecting various numbers of said cells to said armature ofsaid second machine,

a said second switch means including a second principal movable contactand third and fourth auxiliary contacts leading and lagging,respectively, the position of said second principal contact and eachconnected through a diode to said second principal contact.

2. An industrial truck, comprising,

drive wheels,

a hoisting mechanism,

a storage battery comprising a plurality of cells connected in series,

first and second terminals connected to those cells which have maximumpotential difference therebetween,

first and second taps connected to said first and second terminalsrespectively,

a plurality of additional taps connected to intermediate cells of saidbattery, the cells between taps constituting sections, 1

a first direct current dynamo electric machine having an armatureoperating in a substantially constant magnetic field and operativelyconnected to said wheels,

first switch means for selectively connecting various ones of said tapsto said armature of said first machine,

a second direct current dynamo electric machine having an armatureoperating in a substantially constant magnetic field and operativelyconnected to said hoisting mechanism,

second switch means for selectively connecting various ones of said tapsto said armature of said second machine,

an engine,

an alternating current generator driven by said engine,

and

means for charging said battery sections separately with power obtainedfrom said generator,

3. A tap changing system for adjusting the potential applied to a loadfrom a multi-potential direct current source, comprising,

first and second terminals connected to those points of said sourcewhich exhibit maximum potential difference,

first and second taps connected to said first and second terminalsrespectively,

a plurality of additional taps connected to intermediate points on saidsource,

a principal movable contact cooperating with said taps for adjusting thepotential applied to said load,

first and second auxiliary contacts movable simultaneously with saidprincipal contact, one leading and one lagging the position of saidprincipal contact, and

first and second diodes interconnecting said first and second auxiliarycontacts respectively with said principal contact,

said diodes being poled to prevent passage of current between taps.

4, A tap changing system for adjusting the potential applied to a loadfrom a multi-potential direct current source, comprising,

first and second terminals connected to those points of said sourcewhich exhibit maximum potential difference,

a common conductor interconnecting said first terminal of said sourceand one terminal of said load,

first and second taps connected to said first and second terminalsrespectively,

a plurality of additional taps connected to points on 15 plied to a loadfrom a multi-potential direct current source, comprising,

first and second terminals connected to those points of said sourcehaving maximum potential difierence therebetween,

a common conductor interconnecting said first terminal of said sourceand one terminal of said load,

first and second taps connected to said first and second terminalsrespectively,

a plurality of additional taps connected to points on said sourcebearing potentials intermediate those of said first and secondterminals,

a principal contact connected to the other terminal of said load andselectively connectable to any of said taps,

first and second auxiliary contacts movable with said principal contact,

one of said auxiliary contacts making a connection with any tap prior toconnection thereto by said principal contact,

the other of said auxiliary contacts making a connection with any tapsubsequent to connection thereto by said principal contact,

each contact breaking its connection with any tap before making aconnection to the next succeeding tap,

said principal contact retaining its connection with any one tap untilone of said auxiliary contacts has made its connection with the nextsucceeding tap, and

first and second diodes connected between said first and secondauxiliary contacts respectively and said principal contact,

said diodes being poled to prevent short circuiting of any portion ofsaid source as said contacts are actuated.

6. A motor control system, comprising,

a multi-celled battery,

first and second terminals connected to those points on said batterywhich exhibit maximum potential difference therebetween,

first and second taps connected to said first and second terminalsrespectively,

a plurality of additional taps connected to various cells of saidbattery bearing potentials intermediate those of said first and secondterminals,

a principal contact,

first and second auxiliary contacts positioned oneither side of saidprincipal contact,

all of said contacts being mechanically connected together forsimultaneous movement relative to and engagement with said taps,

the width and spacing of said taps and contacts being selected so thateach contact breaks contact with a tap before making contact with thenext adjacent tap and so that two contacts can engage the same tapsimultaneously,

an electric motor,

a connection between said principal contact and said motor, and

first and second diodes connected between said first and secondauxiliary contacts respectively and said motor,

said diodes being poled to prevent passage of current between adjacenttaps.

7. A motor control system, comprising,

a multi-celled battery,

first and second terminals connected to opposite ends of said battery,

first and second taps connected to said first and second terminalsrespectively,

a plurality of additional taps connected to various cells between saidends of said battery,

a common conductor connected to one terminal of said battery,

a principal contact,

first and second auxiliary contacts positioned on either side of saidprincipal contact,

said contacts being fixedly spaced with respect to each other andrelatively movable unitarily with respect to said taps for adjustableengagement therewith,

the width and spacing of said taps and contacts being selected so thateach contact breaks contact with one tap before making contact with thenext adjacent tap and so that two contacts can engage the same tapsimultaneously,

an electric motor having one terminal connected to said commonconductor,

a connection between said principal contact and a second terminal ofsaid motor, and

first and second diodes connected between said first and secondauxiliary contacts respectively and said second terminal of said motor,

said diodes being poled to prevent passage of current between adjacenttaps.

8. A motor control system, comprising,

a storage battery including a plurality of cells connected in series,

first and second terminals connected to opposite ends of said battery,

a common conductor connected to said first terminal of said battery,

first and second taps connected to said first and second terminalsrespectively,

a plurality of additional taps connected to various cells of saidbattery between said first and second terminals,

a dynamo electric machine including an armature and a field structure,

means for supplying a substantially constant magnetic flux to said fieldstructure,

a connection between one terminal of said armature and said commonconductor,

a selectively actuatable switch mechanism interconnecting the otherterminal of said armature and said taps,

said switch mechanism including a principal contact connected to saidother terminal of said armature and selectively connectable to any ofsaid taps,

said switch mechanism also including first and second auxiliary contactsmovable with said principal contact,

one of said auxiliary contacts making a connection with any tap prior toconnection thereto by said principal contact,

the other of said auxiliary contacts making a connec tion with any tapsubsequent to connection thereto by said principal contact,

each contact breaking its connection with any tap before making aconnection to the next succeeding tap,

said principal cont-act retaining its connection with any one tapuntilone of said auxiliary contacts has made its connection with the nextsucceeding tap, and

first and second diodes connected between said first and secondauxiliary contacts respectively and said principal contact,

said diodes being pole-d to prevent short circuiting of said batterycells as said switch mechanism is actuated.

9. A speed control system for an industrial truck including drivewheels, comprising,

a direct current dynamo electric machine including an armature and afield structure mounted on said truck,

means for mechanically connecting said machine to said drive wheels forrotation therewith,

a multi-celled battery mounted on said truck,

first and second terminals connected to those points on said batterywhich exhibit maximum potential difference therebetween,

first and second taps connected to said first and second terminalsrespectively,

a plurality of additional taps electrically connected to various cellsof said battery bear-ing potentials intermediate those of said first andsecond terminals,

means for supplying a substantially constant magnetic flux to said fieldstructure, and

multi-position switch means cooperating with said taps for selecting thevoltage to be applied to said armature, whereby each position of saidswitch means corresponds to a preselected speed of said truck regardlessof whether said truck encounters an up grade or a down grade.

10. A speed control system for an industrial truck including drivewheels, comprising,

a direct current dynamo electric machine including an armature and afield winding mounted on said truck,

means for mechanically connecting said machine to said drive wheels forrotation therewith,

a multi-celled battery mounted on said truck,

first and second terminals connected to opposite ends of said battery,

first and second taps connected to said first and second terminalsrespectively,

a plurality of additional taps electrically connected to various cellsof said battery so as to bear potentials intermediate the potentials ofsaid first and second taps,

multi-position switch means cooperating with said taps for selecting thevoltage to be applied to said armature,

means normally energizing said field winding with a predeterminedvoltage, and

means responsive to an increase in armature current above apredetermined magnitude for increasing the voltage applied to said fieldWinding.

11. A speed control system for an industrial truck including drivewheels, comprising,

a direct current dynamo electric machine including an armature and afield winding mounted on said truck,

means for mechanically connecting said machine to said drive Wheels forrotation therewith,

a multi-celled battery mounted on said truck,

first and second terminals connected to opposite ends of said battery,

first and second taps connected to said first and second terminalsrespectively,

a plurality of additional taps connected to various cells of saidbattery at various potentials intermediate the potentials of said firstand second terminals,

selectively actuatable switch means interconnecting said armature withsaid taps and having an off position in which no voltage is applied tosaid armature and having a plurality of other positions in which variousvolt-ages from said battery are applied to said armature,

an auxiliary contact operated by said switch means,

means for normally energizing said field winding through said auxiliarycontact when said switch means is in any position other than its offposition, and

means responsive to the flow of armature current for energizing saidfield winding regardless of the condition of said auxiliary contact.

12. A speed control system for an industrial truck including driveWheels, comprising,

a direct current dynamo electric machine including an armature and afield winding mounted on said truck,

means for mechani-cally connecting s-aid machine to said drive wheelsfor rotation therewith,

a multi-celled battery m-ounted on said truck,

a common conductor connected to one terminal of said battery,

a plurality of taps, one connected to said common conductor and theremainder connected to various cells of said battery,

a relay including first and second operating solenoids energization ofeither of which actuates said relay,

said relay including a normally open contact,

said first solenoid being connected between said common conductor andone terminal of said armature,

multi-position switch means cooperating with said taps for connecting aselected tap to another terminal of said armature,

whereby when said switch means selects that tap connected to said commonconductor a closed circuit comprising said armature and said firstsolenoid is completed and whereby when said switch means selects anothertap a voltage is applied to said armature,

said field winding having one terminal connected to said commonconductor and the other terminal connected through said normally opencontact to one of said taps,

an auxiliary switch operated by said switch means to its open positionwhen said switch means selects that tap connected t-o said commonconductor and to its closed position when any other tap is selected, and

circuit means connecting one terminal of said second solenoid to saidcommon conductor and the other terminal through said auxiliary switch toone of said t aps,

whereby when said switch means applies a voltage to said alrrnraturesaid second solenoid is energized thereby energizing said field windingand whereby when said switch means selects that tap connected to saidcommon conductor, any counter eleotromotive force generated by saidarmature energizes said first winding thereby maintaining theenerg-ization of said field winding.

References Cited by the Examiner UNITED STATES PATENTS 2,813,984 11/1957Dolecki et al 290-17 X 2,864,047 12/ 1958- Greene 318-338 2,964,69112/1960 Dinger 318-338 3,018,849 1/ 1962: Kush 187-9 3,021,469 2/ 19-62Ganiere 320-15 3,064,7 6 1 11/ 1962 Ramsey 187-9 3,134,063 5/1964Hastings 318-139 3,153,186 10/1964 Medlar 320-15 3,168,688 2/1965-Roggenkamp 318-17 3,179,198 4/1965 Hastings 318-139 3,182,742 5/1965 Dow31 8-139 X 3,188,543 6/1965 Colvill et a1. 318-139 FOREIGN PATENTS1,197,508 7/ 1959 France.

oRrs L. RADER, Primary Examiner.

ANDRES H. NIELSEN, Examiner.

G. SIMMONS, Assistant Examiner.

1. AN INDUSTRIAL TRUCK, COMPRISING, DRIVE WHEELS, A HOISTING MECHANISM,A STORAGE BATTERY COMPRISING A PLURALITY OF CELLS CONNECTED IN SERIES, AFIRST DIRECT CURRENT DYNAMO ELECTRIC MACHINE HAVING AN ARMATUREOPERATING IN A SUBSTANTIALLY CONSTANT MAGNETIC FIELD AND OPERATIVELYCONNECTED TO SAID WHEELS, FIRST SWITCH MEANS FOR SELECTIVELY CONNECTINGVARIOUS NUMBERS OF SAID CELLS TO SAID ARMATURE OF SAID FIRST MACHINE,SAID FIRST SWITCH MEANS INCLUDING A FIRST PRINCIPAL MOVABLE CONTACT ANDFIRST AND SECOND AUXILIARY CONTACTS LEADING AND LAGGING, RESPECTIVELY,THE POSITION OF SAID FIRST PRINCIPAL CONTACT AND EACH CONNECTED THROUGHA DIODE TO SAID FIRST PRINCIPAL CONTACT,